Tectonic Plates and Angular Momentum

As we know, the earth spins on its axis once every twenty four hours.  (Well, actually it doesn’t, but we’ll leave aside the difference between solar and sidereal days for the purpose of this entry). 

The spinning earth posses something we physicists call angular momentum.   It is the ‘spinning’ version of linear momentum;  the latter being the product of a thing’s mass and velocity.  Linear momentum  is a measure of how difficult it is to stop the object by applying a force to it – things with a lot of momentum take a lot of force or a lot of time to stop. Likewise, something spinning with a lot of angular momentum will take a lot of torque, or a lot of time, to bring to a rest. Also, something spinning with its mass distributed far from its axis (an ice skater with her arms outstretched) has greater angular momentum than something spinning with the SAME mass and SAME spin rate,  but with its mass distributed close to the axis (an ice skater with her arms pulled inwards). That’s because it has greater rotational inertia.

In the absence of external forces, linear momentum is conserved (Newton’s first law); likewise in the absence of torques (that is, a twisting force) angular momentum is conserved. Thus the earth keeps spinning, on its own axis, once every 24 hours.

Except that the earth CAN change its spin rate, and its axis of rotation, if somehow the way its mass is distributed moves. This is what appears to have been measured following the Chile earthquake. The movement of the plates appears to have caused a change in the way the mass of the earth is distributed. The movement of one plate under the other has caused a net movement of mass towards the centre of the earth. This gives a decrease in the rotational inertia of the earth, and, as a result, the earth’s rate of spin has increased. Note that angular momentum is conserved here.

It’s the earth-equivalent of the ice-skater increasing her spin rate by pulling her arms in.

How much has the day changed by?  Only about a microsecond.  Not something that you are likely to notice.

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